Microelectromechanical systems (MEMS) are commonly formed by etching components into a thin wafer of silicon. While MEMS devices are much smaller than macroscopic machines, many MEMS devices need moving parts just like their macroscopic counterparts, requiring that some components in the MEMS device be surrounded by free space allowing them to move. Free space allowing a MEMS component to move may be formed by etching trenches into the silicon layer that surrounds components in a MEMS device. Additionally, after some MEMS devices are etched from a silicon wafer, a cap layer of polysilicon material is deposited over top the MEMS to encapsulate the device. This cap layer seals the inner moving parts into an internal cavity, and may route electrical connectors to and from the MEMS devices as well.
Currently, MEMS structures with a high width to depth aspect ratio greater in thickness than a few microns are limited to displacements on the order of a few microns or less when polysilicon deposition layers are used to encapsulate the device perimeter to form a hermetic outer shell. The limitation exists because a layer of oxide, often SiO2, is used as a spacer to separate the polysilicon cap from the underlying components in the MEMS. The oxide layer must span any trenches formed in the silicon or device layer in order to form an even surface on which the polysilicon cap may be deposited. As the width of the trenches increases, the amount of oxide that must be deposited must also increase in order to provide the desired flatness. Thicker oxide layers increase the bulk of a device. Additionally, as oxide layer thickness increase, the stresses imparted onto the underlying wafer by the oxide layer increase. Thus, if a particular oxide layer is too thick, the underlying wafer may crack under the strain.
Because of the foregoing limitations, a typical prior art MEMS hermetic encapsulation process allows for internal trenches of up to about 0.2 μm-1.5 μm in size. Because many MEMS structures must be able to move in order to operate, the small internal trench size limits the range of motion that can be attained using current trench forming techniques. For example, the prior art trench formation techniques allow for the formation of MEMS devices such as capacitative resonators and oscillators that only need to travel within the 0.52 μm-0.58 μm range of existing trenches. Different types of MEMS such as accelerometers or gyroscopes require much larger travel distances and cannot be fabricated in the same generic silicon technology. Typical MEMS vibratory gyroscopes require a movement range on the order of 5 μm-10 μm in order to mechanically amplify angular rate sense mode Coriolis response displacement as a function of drive mode velocity, where {right arrow over (F)}Corolis=2m{right arrow over (ν)}X{right arrow over (Ω)}.
This need for increased range of motion requires the trench width to increase to allow for a range of motion of approximately 5 μm-10 μm in order to provide the desired amount of travel. Using prior art techniques, these wider trenches require an upper oxide layer approximately 10 μm-20 μm in thickness, requiring a large amount of oxide. Moreover, the thick layer generates stress that would likely fracture the underlying silicon wafer. Thus, prior art trench forming methods do not allow for many useful MEMS components like accelerometers and gyroscopes to be manufactured in the manner described above.
What is needed is a method of forming wide area trenches in MEMS devices encapsulated with conformal deposition films. What is further needed is a method for avoiding film stresses in silicon wafers used for fabrication of large displacement MEMS devices.